Sandwich structure mold

ABSTRACT

A mold and fabrication method is disclosed for use in molding precision cast parts. In cross-section the mold has inner and outer layers, the essential ingredients of which include a ceramic refractory material and an inorganic binder. Intermediate layers are disposed between the inner and outer layers and employ an organic fugitive binder. When the mold is heated prior to receiving molten metal, the fugitive binder is volatilized, leaving the intermediate layer unbound. After the parts have been cast the mold is easily collapsed for removal of the casting without damage.

Sept. 9, 1975 SANDWICH STRUCTURE MOLD Inventor: Nicholas G. Lirones,North Muskegon, Mich.

Primary Examiner-Francis S. Husar AXSLVIIUII ExaminerCarl ROWOldAssignee: Howme' 'P GrcenWlChr Armrmy Agent, or F1'rmMcDougall, Hersh &Scott Conn.

[57] ABSTRACT A mold and fabrication method is disclosed for use in [22]Filed: Dec. 26, 1973 [2]] Appl No.: 427,732

molding precision cast parts In cross-section the mold 1 has inner andouter layers, the essential ingredients of [52] U.S. 164/26; l06/38i35;164/36 [51] 822C [/08; 822C l/lO which include a ceramic refractorymaterial and an inorganic binderv Intermediate layers are disposedbetween the inner and outer layers and em ganic Int.

ploy an orfugitive binder. When the mold is heated prior to receivingmolten metal, the fugitive binder is volatilized, leaving theintermediate layer unbound. After the parts have been cast the mold iseasily collapsed a] of the casting without damage.

[56] References Cited UNITED STATES PATENTS for remov lO6/389 X |06/38.3X 26 5 16 Claims, 2 Drawing Figures Opcrhall et al.

INNER LAYER INTERMEDIATE LAYER l4- PATENTEDSEP ems FIG, 2

LPATTERNW L PATTERN I swcco I @PEAfl mNER LAYER INTERMEDIATE.

PRE

ECOND DIP COAT FIRST DIPCOAT PRE HEAT POUR METAL REMOVE MOLD BACKGROUNDOF INVENTION This invention relates to the art of precision casting andto materials employed in the practice thereof. More particularly, theinvention relates to a casting process and to compositions and methodsused in the preparation of molds.

A basic process employed in casting precision parts is known as the lostwax process. This process is partic ularly adapted to producingprecision castings for the production of turbine blades, vanes, as wellas other complex parts. The conventional lost wax process consists ofproducing disposable patterns formed of wax, plastic or other suitablematerial having the desired shape. If necessary, these patterns areassembled into clusters to form a complete assembly. The patterns arethen dipped into a ceramic slurry dipcoat containing a colloidal silicaor other inorganic binder. While the pattern is wet from the dipcoat. itis stuccoed with granular refractory particles and then dried. Thisforms a coating on the patterns and by repeating the dipping, stuccoingand drying steps, a mold of a desired thickness can be formed about thepattern.

The pattern is then removed from the mold and the mold is heated to atemperature whereby the silica binder cures and strongly bonds theceramic dipcoating and granular stucco into a monolithic mass. Afterinspection, the mold is preheated to a high temperature suitable forreceiving molten metal.

After pouring the metal, the casting is allowed to cool to a solidifiedstate. At this stage, the mold strength can be higher than the metalstrength, often resulting in hot tears on the casting. After cooling,the mold is broken away from the casting by mechanical or pneumaticvibration, salt bathing, sand blasting or other techniques.

The above described techniques for separation of the casting from themold have several disadvantages. The high amplitude mechanical vibrationnecessary to break the mold can cause cracking of the casting. Saltbathing techniques and sand blasting are excessively time consuming andalso detrimental to the casting surface in that intergranular attack andsurface erosion occurs.

It is accordingly an object of the present invention to produce andprovide a method for producing new and improved molds for use inprecision casting.

More specifically, it is an object of the present invention to produce amold which is of sufficiently high strength and stability to enablecasting materials to be poured directly therein for molding. and yetwhich can be easily broken away from the casting without damage thereto.

It is another object of the present invention to pro vide a mold inwhich the molded products can be easily and efficiently separated fromthe mold.

It is a further object of the present invention to provide a mold inwhich at least some ofthe materials used therein can be reclaimed andre-used.

It is still a further object of the present invention to provide asandwich structured mold which eliminates casting hot tears" andknock-out cracks.

Other objects of the present invention will become apparent from theconcluding portion of the specificatron.

SUMMARY OF THE INVENTION A mold for casting precision parts is formed bydipping a wax or plastic pattern into a dipcoat of a first compositionincluding finely divided refractory materials and an inorganic binder,such as a silica binder. While the pattern is still wet, it is stuccoedwith granular refractory particles, and then dried. This sequence isrepeated as desired to build up an inner layer ofa desired thickness.The pattern and inner layer are then dipped into a second dipcoat, theprincipal constituents of which are finely divided refractory materialand an organic fugitive binder. After stuccoing, this coating is dried.This intermediate layer may be built up as desired by repeating thesequence.

Finally, an outer layer is formed over the intermediate layer and iscomposed somewhat of the same inorganic constituents as the inner layer,thereby forming a mold which is sandwich structured. When the mold ispreheated prior to metal pouring (after the pattern has been removed),the organic fugitive binder of the intermediate layer is substantiallyeliminated by burning and/or volatilization. This leave the materials ofthe intermediate layer without binder sandwiched between the inner andouter layers.

After molding, the sandwich mold is easily removed from the casting bycollapsing the outer layer onto the intermediate layer and removing thethin inner layer. This mold avoids hot tears as well as cracking in thecasting due to the force heretofore required for mold removal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic sectional viewthrough a pattern having a mold formed thereon in accordance with thepractice of this invention; and

FIG. 2 is a flow diagram of the process embodying the practice of thisinvention.

DETAILED DESCRIPTION The concepts of this invention are embodied in twophases. a mold phase" which includes the compositions employed in themanufacture of the mold and the method for the production thereof, andthe use phase wherein the formed mold is employed in the process ofmolding compositions and materials.

The mold phase will be described with reference to compositions employedand the methods of manufacture in a representative process illustratingthe practice of this phase of the invention.

In the following description, the term pattern will be usedinterchangeably with cluster to refer to a wax or plastic pattern 10 ora cluster formed of a multiplicity of such individual patterns.

The pattern 10 is formed of conventional materials disposable by heat orchemicals. If the mold is to be formed about more than one pattern, theplurality of patterns are connected by runners for communication with apouring spout to form a completed cluster, as described in the issuedUS. patent to Lirones. US. Pat. No. 3.266,IU6. Where the cluster is tobe repeatedly dipped into slurry. identified as a dipcoat, it isdesirable to provide a hanger rod for carrying the cluster and forsuspending the cluster for drying and the like.

First Dipcoating Composition 8000 cc. colloidal silica (30% grade)(specific gravity 165 pounds zircon (99% through 325 mesh) 6150 cc.water 1 grams sodium fluoride Application of First Dipcoat CompositionThe pattern or cluster is first inspected to remove dirt, flakes andother objects which may have adhered to the surfaces of the pattern andwhich, if allowed to remain, would impair the preparation of a good moldand lead to an unacceptable casting. The cleaned clus ter is immersedinto the stirred dipcoat composition to cover all of the surfaces of thecluster. To promote the elimination of air pockets, it is desirable torotate the cluster while immersing in the dipcoat composition.Alternately, the dipcoat composition can be applied by spraying thedipcoat composition onto the surfaces of the pattern.

When fully wet, the pattern or cluster is suspended to drain excessdipcoat composition. During drainage, the cluster can be inspected todetect air pockets which can be eliminated by addressing a stream of aironto the uncoated portions and thereafter allowing the slurry of thedipcoat composition to flow onto the uncovered area. While the clusteris being drained, it should be held in different spatial planes designedto achieve uniform coating on all surfaces. In general, drainage shouldbe completed within a few minutes but, in any event, in less time thanwould allow the dipcoat composition to dry whereby the surface would notretain stucco, as will be described.

First Stucco Coat Stucco combination-Alundum (100% through 50 mesh withless than 3% through 100 meshbetter than 90% between 60 and 80 mesh).

Application of Stucco Coat After the uniformity of coating has beenachieved in the first dipcoat and dripping from the patterns has bccomeminimized. the stucco is sprinkled onto the wet cluster substantiallyuniformly to cover the wet surfaces with a layer of the stucco while, atthe same time, minimizing flow of the dipcoat whereby nonuniformitiesmight otherwise develop. In practice, the stucco particles will berained down from above through a screening member constantly being fedby a vibratory feeder to remove foreign matter from the Alundumparticles, while the particles are sprinkled over an area to give moreuniform and complete coverage. The stucco will adhere to the wet coatingof the slurry and will become partially embedded in the slurry to becomeintegrated with the coating formed on the cluster of wax patterns.

By repeating the above steps, after intermediate drying, the firstdipcoat composition and stucco are built up onto the pattern to form aninner layer l2 of a desired thickness. For the purposes of castingmolten metal products of normal weight, it is desirable to provide formore than one dipcoat-stuccoing cycle. After sufficient drying, thepattern is then emersed into a second dipcoat to form an intermediatelayer 14.

Second Dipcoating Composition Example A. -325 Mesh Alumina Flour 3500 GrPolyvinyl Alcohol (7.2664 aqueous Solution) 1033 Gr Potato Starch (3%Water Solution) 938 Gr Anionic Wetting Agent 4" cc Example B. 325 MeshAlumina Flour 3500 Gr Polyvinyl Alcohol (726% Aqueous Solution) 2000 GrAnionic Wetting Agent 42 cc Example C- 400 Mesh Zircon Flour 4040 GrPolyvinyl Alcohol (7.26% Aqueous Solution) 1033 Gr Potato Starch (3')?Water Solution) 938 Gr Anionic Wetting Agent 42 cc Anti-foam Colloid 21cc Example D.- -400 Mesh Zircon Flour 4040 Gr Polyvinyl Alcohol (7.26%Aqueous Solution) I975 Gr. Anionic Wetting Agent 42 cc Anti-foam Colloid21 cc Example E.

325 Mesh Alumina Flour 122.7 Lbs (Aquadag) Colloidal Graphite (22%Solids) 10.7 Lbs Water (Distilled) 26.3 Lbs Anionic Wetting Agent 418.0cc Gum 'lragacanth Solution 456.0 cc Example F.

325 Zircon Flour 33.0 Lbs (Aquadag) Colloidal Graphite [22% Solids inWater) 8.8 Lbs Water (Distilled) 1 1.0 Lbs Anionic Wetting Agent 200.0cc Gum Tragacanth Solution 300.0 cc Example G.

325 Mesh Zircon Flour 160.0 Lbs Latex 280.0 cc Water 14000 cc GumTragacanth Solution 3000 cc Anionic Wetting Agent 4000 cc ()ctyl Alcohol49.0 cc

ln examples E, F, and G the gum tragacanth solution has the followingcompositions:

Gum Tragacanth Powder 445 Gr Sodium Benzoate S Gr Water 20,000 Gr As theanionic wetting agent, use can be made of sodium heptadecyl sulphate,such as Tergitol Anionic No. 7 of Union Carbide Corporation.

Representative of the antifoam colloids which may be used in the aboveformulations is Antifoam Colloid No. SXlB produced by Colloids, lnc.,Newark, NJ.

Application of Second Dipcoat Composition The second dipcoat compositionis applied to the coated pattern in a manner similar to that of thefirst dipcoat composition. A significant difference between the dipcoatcompositions is in the selection of the binder. The first dipcoatcomposition employs a colloidal silica binder. The second dipcoatcomposition preferably utilizes an organic fugitive binder as, forexample, Polyvinyl alcohol. By organic fugitive binder it is intended todescribe an organic compound which is ca pablc of being volatilizedand/or burned out by heating or otherwise. By employing an organicfugituve binder it is possible to form a second intermediate layer aboutthe pattern. and at a subsequent time drive off the binder to leave theintermediate layer closely packed but unbound.

In the preferred practice of this invention, it is desired, although notessential. to precede the emersion of the coated pattern into the firstand second dipcoat compositions with a pre-wetting step in which theprewetting composition employs substantially the same formulation as thedipcoat composition, with the exception that a lower viscosity isemployed. The prewetting composition includes additional amounts ofwater sufficient to reduce the total solids to about 25 to 75 percent ofthe total solids in the dipcoat composition. Thus, the coated pattern,having the inner layer 12 dried thereon. is submerged into a pre-wettingcomposition more completely to penetrate and wet out the coated surfaceprior to being immersed in the second dipcoat composition. Immersioninto the second dipcoat composition follows immediately after thepre-wet immersion.

After the immersion in the second dipcoat composition, the pattern isagain allowed to drain and is stuccoed and dried. The steps ofpre-wetting, dipcoating, stuccoing and drying are repeated one or moretimes, as necessary, to build up the intermediate layer 14 to a desiredthickness. The stucco employed for the intermediate layer may be thesame as that employed for the inner layer.

For special applications and particularly where large molds areutilized, it is desirable to add a very small percentage, on the orderof 0. l 25 percent by weight, of colloidal silica binder to the seconddipcoat composition which also contains the organic fugitive hinder aspreviously discussed. The presence of the colloidal silica binderprevents the intermediate layer from becoming completely powdery afterthe fugitive binder is driven off as will be described. The small amountof silica binder added to the second dipcoat does not in crease thestrength or increase the task of removal but permits patching and repairof the mold should such operations be necessary. The following exampleillustrates the use of a small amount of silica binder in the seconddipcoat:

Example H.

Zircon Flour [54 Lbs Antifoam Colloid 500 cc Anionic Wetting Agent 720cc Colloidal Silica (309i Sol.) 242 cc Polyvinyl Alcohol (726% Sol.)30.0 Lbs Water i243 Lbs Third Dipcoat Composition and Application Thecomposition of the third dipcoat. which forms the outer layer 16 of themold can, but need not be, the same as the composition of the firstdipcoat. It is applied and stuccoed in a manner similar to that of thefirst dipcoat. However, usually a coarser stucco is applied. When theouter layer l6 has been built up to a desired thickness the mold issubjected to a final drying cycle.

As an example of the number of dipcoats required to form each of thethree layers of the sandwich structure for an average weight casting,two dipcoatings have been employed to form the inner layer 12, threedipcoatings have been employed to form the intermediate layer 14,followed by two dipcoats in the first dipcoat composition to form theouter layer 16. For heavier castings or for large molds, the thicknessof the layers may be increased as necessary.

After final drying, the completed mold is heated to remove the patterncontained therein. and leave a mold cavity in which the material to becast is poured. Pattern removal, hereinafter referred to as dewaxing,can be achieved in a number of ways, as disclosed in US. Pat. No.3,266,l06. Usually dewaxing the mold is accomplished by firing it to ahigh temperature. During such'heating, the colloidal silica binder inthe inner and outer layers 12 and 16 strongly bonds the ceramic dipcoatand granular stucco into a monolithic mass. Simultaneously, the organicfugitive binder in the intermediate layer 14 is volatilized and/orburned out leaving the materials making up the intermediate layerunbonded and sandwiched between the inner and outer bonded layers.

Alternatively, the sandwich structure mold of the present invention canbe dewaxed at a lower tempera ture on the order of 200 to 400 F. Thisdewaxing method is possible due to the high green strength achieved inthe present sandwich structure mold.

lf the sandwich mold is dewaxed at a low temperature. then it will bepreheated to a high temperature prior to use, as indicated in H0. 2, forburning out the mold, i.e., removing any small traces of remaining wax,for burning out or volatilizing the organic binder used for theintermediate layer and for curing the silica binder. Preheating isusually carried out at temperatures in the range of l,000 to 2,850" F.

The sandwich structure mold, after being preheated. is ready forpouringthe molten metal in the mold cavity. During the preheating andpouring of the metal, the sandwich structure mold does not gainsufficient additional strength to cause excessive hardening of the mold.Another advantage of a sandwich structure mold is that, during thecooling process, the relatively thin inner layer 12 is able to collapse,thereby eliminating the possibility. of casting hot tears.

After the casting has cooled completely. the mold is removed. ltsconstruction permits the use of low amplitude mechanical vibration tocrack the thin outer layer 16, which breaks off easily. The intermediatelayer, being unbound, falls away from the mold and, if desired, may bereclaimed for re-use. The portions of the inner layer are similarlyeasily broken away.

The castings resulting from this process are relatively free of moldmaterials, hot tears, and knock-out cracks. Also, sand blasting forfinal cleaning is minimized.

While I have shown and described an embodiment of this invention in somedetail, it will be understood that this description and illustration areoffered merely by way of example, and that the invention is to belimited in scope only by the appended claims.

I claim:

I. A method of producing a recyclable sandwich structure mold about adisposable pattern which is rc moved to define a mold cavity comprisingthe steps of:

a. providing a homogenous first layer by wetting the surface of thepattern with a first dipcoat composition. the essential solids of whichconsist of finely divided refractory material. including a high strengthinorganic binder which is chemically inert to metal to be molded in saidcavity;

h. covering the surface of the pattern, while wet with the firstdipcoat. with a granular refractory stucco;

c. repeating, at least once, steps (a) and (b) with intervening dryingto form a first layer of said mold;

d. wetting said first layer with a second dipcoat composition, theessential ingredients including finely divided refractory material andan organic fugitive binder;

e. covering the surface of the first layer while wet with the seconddipcoat with a granular refractory stucco;

f. repeating, at least once, steps (d) and (c) with in tervening dryingto form an intermediate layer of said mold; and

g. forming an outer layer over said intermediate layer in the mannerrecited in steps (a). (b) and (c). to thereby produce a three layersandwich structure mold.

2. The method of claim 1 further including the step of volatilizing theorganic fugitive binder to leave the intermediate layer unbound andsandwiched between said inner and outer layers.

3. The method of claim I wherein the organic fugitive binder ispolyvinyl alcohol.

4. The method of claim I wherein the organic fugi' tive binder is one ofthe group comprising vegetable gums. starches and polymeric latices.

5. The method of claim I, further including the steps of a. removingsaid pattern; and

b. heating said mold to a temperature sufficient to volatilize saidorganic fugitive binder to leave said intermediate layer unbound.

6. The method of claim 1 further including the steps of heating saidmold to remove said disposable pattern, to volatilize said organicfugitive binder in said intermediate layer, and to fuse said inorganicbinder in said inner and outer layers.

7. The method according to claim 5 wherein the step of removing thepattern is accomplished at a temperature within the range of 200 to 400F., and the step of volatilizing the organic fugitive binder isaccomplished at a temperature within the range of l,00()F. to 2,850 F.

8. A sandwich structure recyclable mold for precision casting having awall structure in cross-section comprising: i

a. a homogenous relatively thin inner layer consisting essentially offinely divided refractory material, including a high strength inorganicbinder which is chemically inert to metals to be molded and granularrefractory stucco;

b. an intermediate layer consisting essentially of finely dividedrefractory material, refractory stucco and an organic fugitive binder;

c. an outer layer identical in composition to said inner layer.

9. The mold of claim 8 wherein prior to use the mold 8 is subjected toheating sufficient to volatilize said organic fugitive binder, tothereby leave the intermediate layer refractory material and stuccounbound and sandwiched between the bound inner and outer layers.

10. The mold of claim 8 wherein the'inner and outer layer refractorymaterials and stucco essentially comprise ceramic flour and binder andceramicstucco.

11. The mold of claim 8 wherein the intermediate layer refractorymaterials essentially comprise alumina or zircon flour. 1

12. The mold of claim 8 wherein the organic fugitive binder is selectedfrom the group comprising vegetable gums, starches, polymeric laticesand polyvinyl alcohol.

13. A method of precision casting materials, such as metal alloys,comprising the steps of:

a. producing a sandwich structure mold about a disposable pattern, saidmold having homogenous inner and outer layers consisting essentially offinely divided refractory material, including a high strength inorganicbinder chemically inert. to said alloys and granular refractory stucco,and an intermediate layer consisting essentially of finely dividedrefractory material, refractory stucco, and an organic fugitive binder;

b. removing said disposable pattern from said mold to provide a moldcavity;

c. firing said mold to a temperature sufficient to volatilize saidorganic fugitive binder to thereby leave said intermediate layer unboundand sandwiched between the bound inner and outer layers;

d. pouring a molten metal alloy into said mold cavity;

e. cooling the mold; and

f. removing the mold from the cast metal alloy.

14. The method of claim 13 wherein the disposable pattern is formed ofwax and steps (b) and (c) are accomplished simultaneously by the heatingof said mold to a temperature within the range of l,000 to 2,850 F.

15. The method of claim 13 wherein step (f) includes the sub-steps of:

a. breaking the outer layer with low amplitude mechanical vibration;

b. permitting the unbound intermediate layer to fall away from the moldand collecting for re-use;

c. removing the inner layer.

16. A sandwich structure mold according to claim 9 wherein saidintermediate layer also includes approximately 0.125 percent by weightof an inorganic binder for preventing said intermediate layer frombecoming completely unbound when said organic fugitive binder isvolatilized.

1. A method of producing a recyclable sandwich structure mold about adisposable pattern which is removed to define a mold cavity comprisingthe steps of: a. providing a homogenous first layer by wetting thesurface of the pattern with a first dipcoat composition, the essentialsolids of which consist of finely diviDed refractory material, includinga high strength inorganic binder which is chemically inert to metal tobe molded in said cavity; b. covering the surface of the pattern, whilewet with the first dipcoat, with a granular refractory stucco; c.repeating, at least once, steps (a) and (b) with intervening drying toform a first layer of said mold; d. wetting said first layer with asecond dipcoat composition, the essential ingredients including finelydivided refractory material and an organic fugitive binder; e. coveringthe surface of the first layer while wet with the second dipcoat with agranular refractory stucco; f. repeating, at least once, steps (d) and(e) with intervening drying to form an intermediate layer of said mold;and g. forming an outer layer over said intermediate layer in the mannerrecited in steps (a), (b) and (c), to thereby produce a three layersandwich structure mold.
 2. The method of claim 1 further including thestep of volatilizing the organic fugitive binder to leave theintermediate layer unbound and sandwiched between said inner and outerlayers.
 3. The method of claim 1 wherein the organic fugitive binder ispolyvinyl alcohol.
 4. The method of claim 1 wherein the organic fugitivebinder is one of the group comprising vegetable gums, starches andpolymeric latices.
 5. The method of claim 1, further including the stepsof a. removing said pattern; and b. heating said mold to a temperaturesufficient to volatilize said organic fugitive binder to leave saidintermediate layer unbound.
 6. The method of claim 1 further includingthe steps of heating said mold to remove said disposable pattern, tovolatilize said organic fugitive binder in said intermediate layer, andto fuse said inorganic binder in said inner and outer layers.
 7. Themethod according to claim 5 wherein the step of removing the pattern isaccomplished at a temperature within the range of 200* to 400* F., andthe step of volatilizing the organic fugitive binder is accomplished ata temperature within the range of 1,000*F. to 2,850* F.
 8. A SANDWICHSTRUCTURE RECYCLABLE MOLD FOR PRECISION CASTING HAVING A WALL STRUCTUREIN CROSS-SECTION COMPRISING: A. A HOMOGENOUS RELATIVELY THIN INNER LAYERCONSISTING ESSENTIALLY OF FINELY DIVIDED REFRACTORY MATERIAL, INCLUDINGA HIGH STRENGTH INORGANIC BINDER WHICH IS CHEMICALLY INERT TO METALS TOBE MOLDED AND GRANULAR REFRACTORY STUCCO, B. AN INTERMEDIATE LAYERCONSISTING ESSENTIALLY OF FINELY DIVIDED REFRACTORY MATERIAL, REFRACTORYSTUCCO AND AN ORGANIC FUNGITIVE BINDER, C. AN OUTER LAYER IDENTICAL INCOMPOSITION TO SAID INNER LAYER.
 9. The mold of claim 8 wherein prior touse the mold is subjected to heating sufficient to volatilize saidorganic fugitive binder, to thereby leave the intermediate layerrefractory material and stucco unbound and sandwiched between the boundinner and outer layers.
 10. The mold of claim 8 wherein the inner andouter layer refractory materials and stucco essentially comprise ceramicflour and binder and ceramic stucco.
 11. The mold of claim 8 wherein theintermediate layer refractory materials essentially comprise alumina orzircon flour.
 12. The mold of claim 8 wherein the organic fugitivebinder is selected from the group comprising vegetable gums, starches,polymeric latices and polyvinyl alcohol.
 13. A method of precisioncasting materials, such as metal alloys, comprising the steps of: a.producing a sandwich structure mold about a disposable pattern, saidmold having homogenous inner and outer layers consisting essentially offinely divided refractory material, including a high strength inorganicbinder chemically inert to said alloys and granular refractory stucco,and an intermediate layer consisting essentially of finely dividedrefractory material, refractory stucco, and an organic fugitive binder;b. removing said disposable pattern from said mold to proVide a moldcavity; c. firing said mold to a temperature sufficient to volatilizesaid organic fugitive binder to thereby leave said intermediate layerunbound and sandwiched between the bound inner and outer layers; d.pouring a molten metal alloy into said mold cavity; e. cooling the mold;and f. removing the mold from the cast metal alloy.
 14. The method ofclaim 13 wherein the disposable pattern is formed of wax and steps (b)and (c) are accomplished simultaneously by the heating of said mold to atemperature within the range of 1,000* to 2,850* F.
 15. The method ofclaim 13 wherein step (f) includes the sub-steps of: a. breaking theouter layer with low amplitude mechanical vibration; b. permitting theunbound intermediate layer to fall away from the mold and collecting forre-use; c. removing the inner layer.
 16. A sandwich structure moldaccording to claim 9 wherein said intermediate layer also includesapproximately 0.125 percent by weight of an inorganic binder forpreventing said intermediate layer from becoming completely unbound whensaid organic fugitive binder is volatilized.